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WHAT HAS CHANGED

Snowfall amounts for the lake effect snow this afternoon into this evening have increased with the western Mohawk now predicted to receive 2 to 4 inches of snow (isolated 4-5" amounts mainly north of the NY Thruway). The lake effect band is now expected to extend as far east Saratoga County into southern Vermont this afternoon into the early evening leading to light snow accumulations.

KEY MESSAGES

1. A band of lake effect snow developing off Lake Ontario this afternoon will likely extend into the western Mohawk Valley and southwest Adirondacks leading to 2 to 4" of snow. Gusty northwest winds reaching 30-35mph also develop regionwide today into this evening and the combination of falling snow and strong gusty winds can lead to low visibility and difficult travel.

2. There is moderate confidence for the potential for a light to moderate snowfall late Monday night through Tuesday, which could impact both the morning and evening commutes as well as any pre-holiday travel.

3. Temperatures continue to trend near to slightly above normal for the second half of the week and while there is low to moderate confidence in additional chance for precipitation for the end of the week (mainly after Christmas), forecast confidence in overall timing and amounts remains low.

DISCUSSION

KEY MESSAGE 1...

A moisture starved cold front will continue tracking eastward early this morning into eastern NY and western New England resulting in light snow showers mainly in the Mohawk Valley and southern Adirondacks where snow amounts range from coatings to 1 inch.

In the wake of the boundary, winds shift to the west and become quite gusty as cold air advection intensifies resulting in deep boundary layer mixing. Expecting max wind gusts to range 30 to 35mph regionwide with probabilistic guidance showing a 50 to 70% chance for gusts to exceed 35mph in the Mohawk Valley, Greater Capital District, the Taconics and western MA. The strong cold air advection, westerly flow, and broad troughing extending from the Great Lakes into the Northeast looks to support a multi-lake connection with high res guidance in good agreement showing a single lake effect band developing off Lake Ontario and extending into the western Mohawk Valley by 15 - 17 UTC today. Forecast soundings show a well mixed boundary layer extending up to 800hPa with strong forcing in the low-levels intersecting the DGZ along with conditional lake induced instability for the afternoon hours. As the band becomes better organized, the HREF even shows 20-30% probability values for hourly snowfall rates to exceed 1". Moderate snowfall rates combined with the gusty winds will lead to lower visibility and potential difficult travel within this band. The gusty westerly (270-280 degree) winds will initially direct the band into southern Herkimer County (mainly north of the NY Thruway) with the band persisting for a few hours early this afternoon before winds veer more to the northwest. This will shift the band southward by 21 - 00 UTC when it reaches the NY Thruway and areas south. Blended in more of the RGEM for QPF amounts which handles lake effect well and the latest forecast snowfall amounts have increased a bit to 2 to 4 inches with isolated 4-5" just north of the NY Thruway in southern Herkimer County. Held off on a winter weather advisory given the limited coverage of 4"+ of snow. As the strong westerly winds veer to 270-290 degrees late this afternoon into the early evening hours, there is increasing agreement among the high res guidance that the lake effect band extends eastward and reaches into Saratoga/Washington County and even into southern VT. However, this looks brief as the winds continue veering this evening and the multi-lake connection weakens so only expecting coating up to 1 inch of snow.

Lake effect snow diminishes this evening as winds become more northwesterly and we lose the favorable fetch off Lake Ontario. Any snow showers become directed into central NY.

KEY MESSAGE 2...

We continue to closely monitor a rather compact and vigorous shortwave and associated developing clipper for late Monday night into Tuesday. There remains more spread in the guidance than we would like to see at this lead time regarding the overall track of the sfc low. The ensemble member pressure clusters from the GEFS continues to suggest the sfc low tracks across the southern half of NY towards southern New England while the pressure clusters from the EPS shows the sfc low tracking well to the north through Ontario into Quebec. Despite the difference in the sfc low track, there is a general consensus that an initial period of isentropic lift supports an organized precipitation shield that tracks from the Ohio Valley northeastward Tuesday morning with thermal profiles supporting all snow. The difference in the storm track are leading to differences in the time of arrival of the snow and its intensity as a further north storm track looks to delay the snow until Tues A.M and suggests lower snowfall amounts while the further south track looks to support stronger forcing/higher snowfall amounts and an earlier arrival time starting Monday night. The timing and snow intensity difference will be important for potential impacts to the Tuesday A.M commute as an earlier arrival time and higher snowfall intensity would lead to more difficult travel conditions. Our forecast sides with the earlier arrival time and therefore shows a widespread 1 to 2 inches by 12 UTC Tues with an additional 1 to 2 inches by 18 UTC. By Tuesday afternoon, the strongest lift escapes to our east and snowfall weakens with sfc temperatures warming slightly above freezing in valley areas. This can lead to a rain/snow in the mid-Hudson Valley but wet-bulbing effects look to keep any additional showers as mainly light snow. Latest NBM probabilistic guidance shows a 20 to 40% chance to exceed 4 inches across the region with the highest probabilities 40-50% in the southern Adirondacks and western MA/southern VT where southwest flow aloft leads to upslope enhancements. These areas therefore have the highest potential for winter weather advisories while other areas may fall shy of the 4" threshold (3" in MA/CT).

KEY MESSAGE 3...

High pressure builds over the Northeast just in time for Christmas Eve giving us mainly quiet weather. Then, large scale ridging building across the southern tier of the CONUS starts to expand towards the Northeast Christmas Day into the end of the week. However, additional shortwaves tracking along its northern periphery increases chances for precipitation. An initial shortwave on Christmas Day looks to track mainly to our south and probability for precipitation has therefore lowered to slight chance. Even if precip does reach our southern areas, temperatures looks warm enough to favor rain. Then, chances for precipitation increase for Thurs night into Friday as another shortwave tracks through Canada; however, the latest ensemble cluster analysis shows just 35% of the members favor deepening heights during this window while 65% show increasing heights and therefore a drier forecast. Therefore, the 50th percentile 24hr precip amounts through Fri evening generally remain under a quarter inch with probability for 2" or more of snow under 20%. Warmer air aloft nudges northeastward during this period due to the building ridge and southwest flow so should precipitation chances increase, we will have to monitor precipitation types. Overall forecast confidence for the end of the week remains low.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Through 12z Monday...VFR conditions persist across all terminals this morning, though not for long as the aforementioned cold front approaches from the northwest with a line of low stratus clouds as currently seen on infrared satellite. Ceilings generally should remain within VFR thresholds, though low VFR thresholds, outside of showers but some light snow showers accompanying the fropa could drop conditions into the MVFR category at KALB, KGFL, and KPSF between 13-18z. The passage of this front will also reinforce strong wind gusts throughout the region, quickly backing the current south-southwesterly winds to the west-northwest. Sustained speeds will reach 15-20kt today with gusts of 35-35kt.

KGFL should remain dry after an initial batch of snow showers move through this morning and KPOU will likewise remain dry throughout the duration of the 12z cycle. KALB and KPSF have the potential to see an additional period of snow this afternoon as a lake effect band extends far inland and upslope flow drives snow generation. This will be the better opportunity for IFR conditions, primarily with visibility, but confidence was not high enough to include this in a TEMPO. Therefore, PROB30 groups were added such that more detail can be provided with future updates.

Once snow concludes at KALB and KPSF, VFR conditions will return quickly with winds remaining elevated through at least the first half of the night. But by the end of the period, gusts should be lost and lighter winds should prevail.

Outlook...

Monday Night: Moderate Operational Impact. Likely SN. Tuesday: High Operational Impact. Definite SN. Tuesday Night: Moderate Operational Impact. Breezy. Chance of SHSN. Wednesday: Low Operational Impact. Breezy. NO SIG WX. Wednesday Night: Low Operational Impact. Slight Chance of SHSN. Christmas Day: Low Operational Impact. NO SIG WX. Thursday Night: Moderate Operational Impact. Chance of SHRA...SHSN. Friday: Moderate Operational Impact. Chance of RA...SN.

ALY WATCHES/WARNINGS/ADVISORIES

CT...None. NY...None. MA...None. VT...None.